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16.5. Filtering Your Own Output
16.5.1. Problem
You want to postprocess your program's
output without writing a separate program to do so.
16.5.2. Solution
Use the forking form of open to attach a filter to
yourself. For example, this will restrict your program to a hundred
lines of
output:head(100);
while (<>) {
print;
}
sub head {
my $lines = shift || 20;
return if $pid = open STDOUT, "|-";
die "cannot fork: $!" unless defined $pid;
while (<STDIN>) {
print;
last unless --$lines;
}
exit;
}
16.5.3. Discussion
It's easy to add an output filter. Just
use the forking open on your own
STDOUT, and let the child filter
STDIN to STDOUT, performing
whatever alterations you care about. Notice that we install the
output filter before we generate the output.
This makes sense—you can't filter your output if it has already
left your program. Any such filters should be applied in LIFO
order—the last one inserted is the first one run.Here's an example that uses two output filters. One numbers lines;
the other quotes the lines like a mail reply. When run on
/etc/motd, you get something like:1: > Welcome to Linux, version 2.0.33 on a i686
2: >
3: > "The software required `Windows 95 or better',
4: > so I installed Linux."
If you reversed the order of the two filters, you'd get:> 1: Welcome to Linux, Kernel version 2.0.33 on a i686
> 2:
> 3: "The software required `Windows 95 or better',
> 4: so I installed Linux."
The program is in Example 16-1.
Example 16-1. qnumcat
#!/usr/bin/perl
# qnumcat - demo additive output filters
number( ); # push number filter on STDOUT
quote( ); # push quote filter on STDOUT
while (<>) { # act like /bin/cat
print;
}
close STDOUT; # tell kids we're done--politely
exit;
sub number {
my $pid;
return if $pid = open STDOUT, "|-";
die "cannot fork: $!" unless defined $pid;
while (<STDIN>) { printf "%d: %s", $., $_ }
exit;
}
sub quote {
my $pid;
return if $pid = open STDOUT, "|-";
die "cannot fork: $!" unless defined $pid;
while (<STDIN>) { print "> $_" }
exit;
}
As with all process forks, doing this a zillion times has some cost,
but it's fine for a couple of processes, or even a couple dozen. If
the system was actually designed to be multitasking right from the
start, as Unix was, this is far cheaper than you might imagine.
Virtual memory and copy-on-write makes this efficient. Forking is an
elegant and inexpensive solution to many, if not most, multitasking
needs.
16.5.4. See Also
The open function in Chapter 29 of
Programming Perl and in
perlfunc(1); Recipe 16.4
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| 16.4. Reading or Writing to Another Program |  | 16.6. Preprocessing Input |
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